Finely chop half the onion and slice the other half into rings. Mix together all the ingredients for the burgers, leaving out the onion rings. Set aside to rest for at least 10 minutes in the fridge.
Preheat the oven to 200 C, Gas 6. Mould the mince into 4 burgers and place on an oiled baking sheet. Place in the oven and cook for 10 minutes, turning halfway through the cooking time.
Meanwhile mix the mayonnaise with the coriander and mint leaves, lemon, salt and pepper. Warm the buns in the oven for the final 2 minutes of cooking time.
Place a small handful of the lettuce on the bottom of each bun. Add sliced tomato, onion rings and a good dollop of the mayonnaise. Top with the hot burgers and finish with the top half of the bun. Serve immediately.